The purpose of providing a hollow shaft means, through which a power takeoff shaft means passes, between a clutch housing and transmission casing in the fashion set forth above is to enable an engine to drive the vehicle and an auxiliary implement such as a rotary tiller, mower or the like in manners different from each other. When a dual clutch is employed as a main clutch, it is fashioned such that transmission of the vehicle-driving power is cut off by a first-stage operation of the clutch and, then, transmission of the auxiliary implement-driving power is cut off by a second-stage operation of the clutch. When a separate power takeoff clutch is employed as shown, for example, in JP, A(U) No. 56-97128, a main clutch which can cut off only the transmission of vehicle-driving power is employed and a primary drive shaft in the implement-driving power transmission path is connected directly to the engine.
Tractors are used in conjunction with auxiliary implements for various purposes such as earth-moving, mowing or reaping, article-loading purposes and, thus, various transmissions of different designs are required. An economical way of providing different designs to tractor transmissions is that an additional transmission or speed-change mechanism is provided within a rear half of a clutch housing which includes a substantial dead space behind a main clutch. There have been proposed various structures for such additional transmission mechanism.
These prior art structures may be divided into two types. One of these types employs an additional casing as a support for an additional speed-change mechanism as shown, for example, in JP, A(U) No. 59-37131. The other type employs a support frame which is fixedly mounted in the clutch housing for supporting an additional speed-change mechanism or its transmission shafts, as shown, for example, in JP, A(U) No. 58-22557.
The structure of the former type involves a problem that it is relatively large in weight due to employment of the additional casing so that it is relatively hard to assemble same into the clutch housing. Further, an additional speed-change mechanism of a different design requires a different casing. Such casing is relatively large in size and, therefore, relatively expensive in cost.
The structure of the latter type employing a support frame is small in weight and, therefore, permits an easy assemblage into the clutch housing of a tractor. Cost of this structure is considerably low as compared to the structure employing an additional casing. However, in the prior art structure of this type, a single support frame is employed and transmission shafts are supported by the support frame and by a rear wall of the clutch housing. Such single support frame often requires, when the design of an additional speed-change mechanism is changed, a clutch housing having a rear wall of a different design.